It is proposed to apply mass spectrometry in structural and analytical problems involving nucleosides and their analogs. Mass chromatography and multiple ion detection techniques will be used in conjunction with stable isotope labeling in studies involving metabolism of the antitumor drug 1-(tetrahydro-2-furanyl)-5-fluorouracil in the mouse. Oligonucleotides and modified nucleosides from Novikoff hepatoma will be examined; studies of the mass spectra of nucleoside analogs will be carried out.